Ling Bi-Li

Density Profile and Fluctuation Measurements by Microwave Reflectometry on EAST

A microwave reflectometry system operating in the V-band frequency with extraordinary mode polarization has been developed on the EAST tokamak. The reflectometry system, using a voltage-controlled oscillator (VCO) source driven by an arbitrary waveform generator with high temporal resolution, can operate for the density profile measurement. The result of the bench test shows that the output frequency of the VCO has a linear dependence on time. The dispersion of reflectometry system is determined and reported in this paper. The evolution of a pedestal density profile during the L-H transition is observed by the reflectometry in H-mode discharges on EAST tokamak. A frequency synthesizer is used to replace the VCO as microwave source for density fluctuation measurements. The level of density fluctuation in the pedestal shows an abrupt decrease when the plasma enters into H-mode. A coherent mode with a frequency of about 100 kHz is observed and the mode frequency decreases gradually as the pedestal evolves.

Runaway Electron Beam Instability in Slide-Away Discharges in the HT-7 Tokamak

Slide-away discharges are achieved by decreasing the plasma density or ramping down the plasma current in runaway discharges in the HT-7 tokamak. In the case of plasma current ramp down, the ratio of the electron plasma frequency to the electron cyclotron frequency is higher than in the stationary pulses when the discharge goes into a slide-away regime. The instability regime is characterized by relaxations in the electron cyclotron emission due to relativistic anomalous Doppler effect which transfers energy from parallel to perpendicular motion. The triggering of relativistic anomalous Doppler effect at higher density by ramping down of plasma current may provide a alternative runaway energy control scenario.

Self-Organized Criticality Properties of the Turbulence-Induced Particle Flux at the Plasma Edge of the HT-6M Tokamak

The power spectrum and the probability distribution function (PDF) of the turbulence-induced particle flux Γ in the velocity shear layer of the HT-6M edge region have been measured and analysed. Three regions of frequency dependence (f 0, f-1, f-4) have been observed in the spectrum of the flux. The PDF of the flux displays a Γ-1 scaling over one decade in Γ. Using the rescaled-range statistical technique, we find that the degree of the self-similarity (Hurst exponent) of the particle flux in the measured region ranges from 0.64 to 0.83. All of these results may mean that the plasma transport is in a state characterized by self-organized criticality.

Present Status of the Electron Cyclotron Emission Measurements on HT-7 and EAST

Present status of the diagnostics for electron cyclotron emission measurements on both HT-7 and EAST is reported. A 16-channel heterodyne radiometer system and a 20-channel grating polychromator, have been installed on HT-7 and EAST. A 32-channel heterodyne radiometer system, with a wider frequency coverage and better spatial resolution, is being built. In order to provide a reliable electron temperature profile, an in-situ absolute calibration system is currently being built. With the ECE measurements, study on the electron heat transport and the anomalous Doppler resonance is conducted.

A Signature of Self-Organized Criticality in the HT-6M Edge Plasma Turbulence

Power spectra of electron density and floating potential fluctuations in the velocity shear layer of the HT-6M edge region have been measured and analysed. All the spectra have three distinct frequency regions with the spectral decay indices typical of self-organized criticality systems (0, -1 and -4) when Doppler shift effects induced by the plasma E×B flow velocity have been taken into account. These results are consistent with the predictions of the self-organized criticality models, which may be an indication of edge plasma turbulence in the HT-6M tokamak evolving into a critical state independent of local plasma parameters.

Measurement of Magnetic Island Width by Multi-Channel ECE Radiometer on HT-7 Tokamak ⁄

A 16-channel electron cyclotron emission (ECE) radiometer has been employed to observe the (m;n) = (2;1) magnetic island structure on HT-7 tokamak, where m and n represent the poloidal and toroidal mode number respectively. The results indicate that the island width is about 7 cm when the magnetic island is saturated during the m=n = 2=1 mode. The location of resonance surface can be determined by plotting the contour of ECE relative ∞uctuation. This method could be applied to the HT-7 and EAST campaigns in the future for the research of neoclassical tearing modes (NTMs).

Plasma Boundary Position Measurements Using Frequency Sweep Microwave Reflectometer in LHD

A broadband frequency tunable microwave reflectometer system, which has the ability of fast and stable frequency sweeping operation, is applied in the large helical device (LHD) to measure the boundary of the high-temperature plasma. When the microwave is launched, with extraordinary polarization, from the low field side of the magnetic field with its frequency swept from low to high, the microwave of minimum right-hand cut-off frequency is reflected most outside. We can then estimate the plasma boundary position by measuring the change of the reflected power from the cut-off layer.

Electron Heat Diffusivity Measurement by Perturbative Method on HT-7 Tokamak

Spatio-temporal evolutions of the sawteeth activities are measured simultaneously with a 16 channel high spatio-temporal resolution electron cyclotron emission (ECE) heterodyne radiometer on HT-7 and the evolutions of the sawteeth are found to be diffusive in character. By a perturbative analysis, the electron heat diffusivity χe inferred by time-to-peak method and Fourier transform shows a good agreement. The value of electron heat diffusivity shows radial asymmetry between the low magnetic field side (LFS) and the high magnetic field side (HFS). It is observed that with the increase in plasma density, the electron heat diffusivity decreases and the confinement of energy is improved. By comparing with χpbe inferred by static power balance analysis, the result indicate that χhpe is larger than χpbe by a factor ranging from 2 to 10.

Reynolds Stress and Sheared Poloidal Flow in the Edge Plasma Region of the HT-6M Tokamak

High spatial resolution measurements of the electrostatic Reynolds stress, radial electric field and poloidal phase velocity of fluctuations in the edge region of the HT-6M tokamak are carried out. The Reynolds stress shows a radial gradient in proximity to the poloidal velocity shear. A comparison of the profiles between the Reynolds stress gradient and the poloidal velocity damping reveals some similarity in their magnitude and radial structure. These facts suggest that the turbulence-induced Reynolds stress may play a significant role in generating the poloidal flow in the plasma edge region.

Experimental evidence of the self-similarity and long-range correlations of the edge fluctuations in HT-6M tokamak

For a better understanding of long timescale transport dynamics, the rescaled range analysis techniques, the autocorrelation function (ACF) and the probability distribution function (PDF) are used to investigate long-range dependences in edge plasma fluctuations in an HT-6M tokamak. The results reveal the self-similar characters of the electrostatic fluctuations with self-similarity parameters (Hurst exponent) ranging from 0.64 to 0.79, taking into consideration the r× rotation-sheared effect. Fluctuation ACFs of both the ion saturation current and the floating potential, as well as PDF of the turbulence-induced particle flux, have two distinct timescales. One corresponds to the decorrelation timescale of local fluctuations (µs) and the other lasts to the order of the confinement time (ms). All these experimental results suggest that some of the mechanisms of the underlying turbulence are consistent with plasma transport as characterized by self-organized criticality (SOC).